The operation of expansion machines, such as steam turbines, is known in prior art, for example with the aid of the Organic Rankine Cycle (ORC) method for generating electric energy by employing organic media, for example organic media having low vaporization temperatures, which generally have higher vaporization pressures at the same temperatures compared to water as the operating medium. ORC plants constitute the realization of the Clausius Rankine cycle where electric energy is, for example, in principle obtained by adiabatic and isobaric changes of the state of an operating medium. By means of the vaporization, expansion and subsequent condensation of the operating medium, mechanical energy is obtained here and converted into electric energy. In principle, the operating medium is brought to the operating pressure by a feed pump, and energy in the form of heat provided by combustion or by a flow of waste heat is supplied to it in a vaporizer. The operating medium flows from the vaporizer via a pressure pipe to an expansion machine where it is expanded to a lower pressure. Subsequently, the expanded operating medium steam flows through a condenser where heat exchange takes place between the vaporous operating medium and a cooling medium, whereupon the condensed-out operating medium is returned to the vaporizer by a feed pump in a cyclic process.
A particular class of expansion machines is constituted by volumetrically operating expansion machines which are also referred to as displacement expansion machines and comprise a working chamber and perform work during a volume increase of this working chamber during the expansion of the operating medium. These expansion machines are realized, for example, in the form of piston expansion machines, screw expansion machines, or scroll expanders. Such volumetrically operating expansion machines are in particular employed in ORC plants of small power classes (e. g. with an electrical power of 1 to 500 kW). In contrast to turbines, volumetrically operating expansion machines, however, require lubrication by a lubricant in particular of the piston or of the profiles of the expansion room that roll on each other and of the rolling bearings and the sliding walls of the working chamber. The use of a lubricant advantageously also leads to a sealing of the working area of the expansion machine, whereby less steam is lost due to an overflow within the expansion machine, thus increasing efficiency.
FIG. 1 represents a schematic diagram of a lubrication system of prior art. An operating medium is supplied from a vaporizer 1 to an expansion machine 2. In the expansion machine 2, the vaporous operating medium is expanded, and via a generator 3, the released energy is converted into electric energy. Via a rotary oil pump 4, a lubricant, for example lubricating oil, is supplied to the expansion machine 2. The lubricant exits from the expansion machine 2 together with the expanded operating medium. The lubricant is present in the expanded operating medium in the form of a finely distributed oil mist and is separated from the operating medium in an oil separator 5, so that the operating medium is supplied from the oil separator 5 to a condenser 6 essentially free from oil. The condensed operating medium is supplied again to the vaporizer 1 by a feed pump 7. The recovered oil is supplied again to the expansion machine 2 via the rotary oil pump 4.
The lubrication system of prior art, however, involves the following disadvantages. Since the lubricant (lubricating oil) is separated on the low-pressure side after having passed the expansion machine 2, it is necessary to provide the rotary oil pump 4 which, since the lubricant must be supplied to the expansion machine 2 on the high-pressure side, must overcome the same pressure differential as the feed pump 7 transporting the operating medium, thereby requiring a lot of equipment and causing corresponding high costs. Moreover, a relatively large oil separator 5 is needed as the waste steam exiting from the expansion machine 2 has a lower density compared to the live steam supplied to the expansion machine 2, for example a density that is lower by more than one dimension. Furthermore, the separation of the lubricant from the waste steam of the operating medium is accomplished by means of cyclone separators or deflectors, always involving significant changes of direction of the flow of waste steam containing the lubricant, whereby pressure losses occur in combination with the relatively large volumes of the flow of waste steam, leading to a counter pressure acting on the expansion machine 2 and thus to a decrease in the efficiency of the latter.
Moreover, due to the relatively large mass or the relatively large volume of the waste steam, the relatively large oil separator 5 has a certain inertia having disadvantageous effects when the plant is being started or during changes of loads. Moreover, the lubricant injected by nozzles into the live steam inter alia in a liquid state approx, at the temperature of the waste steam undesirably reduces the live steam temperature and live steam enthalpy.
Thus, there is a demand for this, and it is thus the object of the present invention to provide a method for lubricating volumetrically operating expansion machines in which the above mentioned problems are eliminated or at least attenuated.